1. Field of the Invention
The present invention relates to a feedforward amplifier in which distortion arising due to the nonlinearity of an amplifier is detected in a distortion detection loop and the distortion is cancelled in a distortion elimination loop by using the detected distortion. This kind of amplifier is particularly effective for the cancellation of distortion arising in a multi-carrier amplifier where multiple carriers are simultaneously amplified by using a single amplifier, such as a transmit amplifier used, for example, in a base station of a digital mobile telephone system.
2. Description of the Related Art
In a feedforward amplifier having a distortion detection loop and a distortion elimination loop, the distortion detection loop detects distortion components by combining a portion of the input to an amplifier and a portion of the output from the same amplifier after adjusting their phase and amplitude so that they are opposite in phase and equal in amplitude to each other with regard to the main signal, and the distortion elimination loop cancels the distortion components contained in the output of the amplifier by combining the output of the distortion detection loop with the output of the amplifier after adjusting their phase and amplitude so that they are opposite in phase and equal in amplitude to each other with regard to the distortion. To compensate for changes over time in the amount of attenuation and the amount of phase shifting in a variable attenuator and a variable phase shifter used to provide the opposite phase with equal amplitude, the main signal and a pilot signal inserted at an appropriate point for detection of loop equilibrium are detected from the respective outputs of the distortion detection loop and distortion elimination loop, and the variable attenuator and variable phase shifter in each loop are automatically controlled to minimize the respective amounts. For automatic control of the amount of attenuation and amount of phase shifting in each loop, a perturbation method is employed in which each parameter is changed in a steplike manner with a specified periodicity and with a prescribed perturbation width, the change being continued in the same direction if an improvement is obtained and being reversed in the opposite direction if the situation worsens, thereby finding optimum values.
A detector for detecting the main signal in the distortion detection loop and a detector for detecting loop equilibrium in the distortion elimination loop are each designed with an input dynamic range that is usually determined by considering the largest and smallest input levels that can occur. Further, for the perturbation step width, a sufficiently small step width is usually employed so that the required accuracy (the amount of cancellation) can be obtained.
If, of the above detectors, the detector for detecting the main signal in the distortion detection loop is taken as an example, its input level becomes the largest when the number of carriers contained in the input to the amplifier is the largest and the power of each carrier is also the largest, and when the input and output of the amplifier are combined in phase with regard to the main signal, and becomes the smallest when the number of carriers and the carrier power are both the smallest and when the amplifier input and outputs are combined in opposite phase and equal amplitude with regard to the main signal; therefore, the dynamic range at the input to the detector is wide, for example, 50 dB or more. Accordingly, determining the input dynamic range by considering the largest and smallest possible levels as in the prior art involves the problem that when the number of carriers and the carrier power are small, minutes changes in detection amount near optimum values cannot be detected and deviations from the optimum amplitude and phase values become large, making it impossible to provide a sufficient cancellation amount. If the input range is determined by giving priority to accuracy at the minimum input level, then the detector will saturate and control will become unstable when the number of carriers and the carrier power are large.
For the perturbation step width, if the step width is set small for high accuracy as in the prior art, there arises the problem that it takes time from the moment the control is initiated until the control settles down. Furthermore, if the perturbation width is small, there arises the further problem that the perturbation tends to be performed in the wrong direction because of modulation-induced input signal variations, noise, etc. This also results in unstable control.
The above-outlined problems also apply to the distortion elimination loop.